This invention relates generally to pressure responsive electrical switches and more particularly to such switches which have little differential between actuation and deactuation pressures.
Pressure switches are used for many different applications to monitor pressure conditions and upon occurrence of predetermined changes in pressure to provide a signal of such change or to energize or deenergize some related function.
One type of switch, such as that shown and described in U.S. Pat. No. 4,296,287, has had wide acceptance and has been used for many applications utilizes a snap acting monometallic or multimetallic disc which is welded to a housing thereby enclosing a chamber which is in communication with a fluid medium whose pressure is to be monitored. A switch is mounted on the other side of the disc and comprises a motion transfer member slidably mounted adjacent the disc and extending between the disc and a movable contact arm. When a selected pressure condition occurs, the disc snaps from a first dished configuration to an opposite dished configuration concomitantly transferring motion through the motion transfer member to the movable contact arm to open or close an electric circuit. The disc is adapted to snap in one direction upon being subjected to a particular pressure level or higher and will snap back in the opposite direction to its original configuration when the pressure decreases to a second, lower pressure. This pressure differential is desirable for many applications but is undesirable for certain other applications.
For example, in monitoring the pressure of transmission fluid for off road construction vehicles little or no differential between the actuation pressure and the release pressure is preferred. In such applications a snap acting disc is unsuitable because it inherently has too much pressure differential.
Other known pressure responsive switches such as that shown in U.S. Pat. No. 4,342,887 have a pressure chamber mounted adjacent to a switch with a flexible organic membrane used to seal the chamber and transfer the pressure to the switch. Use of such material for a seal for the present application however is unsuitable because the expected seal life is too short. The present application for transmission fluid monitoring involves relatively high pressure, for example 135 psi, over several hundred of thousands of pressure cycles.
Yet another prior art switch is shown in U.S. Pat. No. 4,272,660 and comprises a switch assembly attached to a vacuum chamber closed by a metal diaphragm. The diaphragm is mounted on a toroidal projection to enable it to flex and transmit pressure to the switch. However the use of the projection tends to introduce stresses in the diaphragm which limits its useful life.
It is therefor an object of the present invention to provide a pressure switch having little or no differential between actuation and release pressure levels. Another object is the provision of a pressure sensor which is hermetically sealed, is resistant to various chemical fluids and has an improved product life. Yet another object is the provision of a pressure responsive switch which is adaptable to a broad range of pressure settings, can be easily adapted to different ranges of pressure and is easily adjustable for fine adjustments. Yet another object is the provision of a pressure switch which is insensitive to vibration, is suitable for use with positive as well as negative pressures, is reliable, long lasting and yet inexpensive.
Other objects, advantages and details of the pressure responsive device of this invention appear in the following detailed description of preferred embodiments of the invention.